The invention relates to the separation of gases from hydrocarbon gas mixtures, such separations including hydrogen from hydrocarbons, carbon dioxide from hydrocarbons, and hydrocarbons from one another. The separation is carried out using hydrocarbon-resistant membranes, and is useful in refineries, petrochemical plants, natural gas fields and the like.
Polymeric gas-separation membranes are well known and are in use in such areas as production of oxygen-enriched air, production of nitrogen from air, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from air or nitrogen.
The preferred membrane for use in any gas-separation application combines high selectivity with high flux. Thus, the membrane-making industry has engaged in an ongoing quest for polymers and membranes with improved selectivity/flux performance. Many polymeric materials are known that offer intrinsically attractive properties. That is, when the permeation performance of a small film of the material is measured under laboratory conditions, using pure gas samples and operating at modest temperature and pressure conditions, the film exhibits high permeability for some pure gases and low permeability for others, suggesting useful separation capability.
Unfortunately, gas separation in an industrial plant is seldom so simple. The gas mixtures to which the separation membranes are exposed may be hot, contaminated with solid or liquid particles, or at high pressure, may fluctuate in composition or flow rate or, more likely, may exhibit several of these features. Even in the most straightforward situation possible, where the gas stream to be separated is a two-component mix, uncontaminated by other components, at ambient temperature and moderate pressure, one component may interact with the membrane in such a way as to change the permeation characteristics of the other component, so that the separation factor or selectivity suggested by the pure gas measurements cannot be achieved. In gas mixtures that contain condensable components, it is frequently, although not always, the case that the mixed gas selectivity is lower, and at times considerably lower, than the ideal selectivity. The condensable component, which is readily sorbed into the polymer matrix, swells or, in the case of a glassy polymer, plasticizes the membrane, thereby reducing its selective capabilities. A technique for predicting mixed gas performance under real conditions from pure gas measurements with any reliability has not yet been developed.
A good example of these performance problems is the separation of hydrogen from mixtures containing hydrogen, methane and other hydrocarbons. Increasing reliance on low-hydrogen, high-sulfur crudes, coupled with tighter environmental regulations, has raised hydrogen demand in refineries. This is primarily due to increased hydrodesulfurization and hydrocracking; as a result many refineries are now out of balance with respect to hydrogen supply. At the same time, large quantities of hydrogen-containing off-gas from refinery processes are currently rejected to the refinery""s fuel gas systems. Besides being a potential source of hydrogen, these off-gases contain hydrocarbons of value, for example, as liquefied petroleum gas (LPG) and chemical feedstocks.
The principal technologies available to recover hydrogen from these off-gases are cryogenic separation, pressure swing adsorption (PSA), and membrane separation. Membrane gas separation, the newest, is based on the difference in permeation rates of gas components through a selective membrane. Many membrane materials are much more permeable to hydrogen than to other gases and vapors. One of the first applications of gas separation membranes was recovery of hydrogen from ammonia plant purge streams, which contain hydrogen and nitrogen. This is an ideal application for membrane technology, because the membrane selectivity is high, and the feed gas is clean (free of contaminants, such as heavier hydrocarbons). Another successful application is to adjust hydrogen/carbon monoxide or hydrogen/methane ratios for synthesis gas production. Again, the feed gas is free of heavy hydrocarbon compounds.
Application of membranes to refinery separation operations has been much less successful. Refinery gas streams contain contaminants such as water vapor, acid gases, olefins, aromatics, and other hydrocarbons. At relatively low concentrations, these contaminants cause membrane plasticization and loss of selectivity. At higher concentrations they can condense on the membrane and cause irreversible damage to it. When a feedstrearn containing such components and hydrogen is introduced into a membrane system, the hydrogen is removed from the feed gas into the permeate and the gas remaining on the feed side becomes progressively enriched in hydrocarbons, raising the dewpoint. For example, if the total hydrocarbon content increases from 60% in the feed gas to 85% in the residue gas, the dewpoint may increase by as much as 25xc2x0 C. or more, depending on hydrocarbon mix. Maintaining this hydrocarbon-rich mixture as gas may require it to be maintained at high temperature, such as 60xc2x0 C., 70xc2x0 C., 80xc2x0 C. or even higher, which is costly and may itself eventually adversely affect the mechanical integrity of the membrane. Failure to do this means the hydrocarbon stream may enter the liquid-phase region of the phase diagram before it leaves the membrane module, and condense on the membrane surface, damaging it beyond recovery. Even if the hydrocarbons are kept in the gas phase, separation performance may fall away completely in the presence of hydrocarbon-rich mixtures. These issues are discussed, for example, in J. M. S. Henis, xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranesxe2x80x9d Chapter 10 of D. R. Paul and Y. P. Yampol""skii, Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. This reference gives upper limits on various contaminants in streams to be treated by polysulfone membranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation of aromatics, 25% saturation of olefins and 11xc2x0 C. above paraffin dewpoint (pages 473-474).
A great deal of research has been performed on improved membrane materials for hydrogen separation. A number of these materials appear to have significantly better properties than the original cellulose acetate or polysulfone membranes. For example, modem polyimide membranes have been reported with selectivity for hydrogen over methane of 50 to 200, as in U.S. Pat. Nos. 4,880,442 and 5,141,642. Unfortunately,these materials appear to remain susceptible to severe loss of performance through plasticization and to catastrophic collapse if contacted by liquid hydrocarbons. Several failures have been reported in refinery applications where these conditions occur. This low process reliability has caused a number of process operators to discontinue applications of membrane separation for hydrogen recovery.
Another example of an application in which membranes have difficulty delivering and maintaining adequate performance is the removal of carbon dioxide from natural gas. Natural gas provides more than one-fifth of all the primary energy used in the United States, but much raw gas is xe2x80x9csubqualityxe2x80x9d, that is, it exceeds the pipeline specifications in nitrogen, carbon dioxide and/or hydrogen sulfide content. In particular, about 10% of gas contains excess carbon dioxide. Membrane technology is attractive for removing this carbon dioxide, because many membrane materials are very permeable to carbon dioxide, and because treatment can be accomplished using the high wellhead gas pressure as the driving force for the separation. However, carbon dioxide readily sorbs into and interacts strongly with many polymers, and in the case of gas mixtures such as carbon dioxide/methane with other components, the expectation is that the carbon dioxide at least will have a swelling or plasticizing effect, thereby adversely changing the membrane permeation characteristics. These issues are again discussed in the Henis reference cited above.
In the past, cellulose acetate, which can provide a carbon dioxide/methane selectivity of about 10-20 in gas mixtures at pressure, has been the membrane material of choice for this application, and about 100 plants using cellulose acetate membranes are believed to have been installed. Nevertheless, cellulose acetate membranes are not without problems. Natural gas often contains substantial amounts of water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. However, contact with liquid water can cause the membrane selectivity to be lost completely, and exposure to water vapor at relative humidities greater than about 20-30% can cause irreversible membrane compaction and loss of flux. The presence of hydrogen sulfide in conjunction with water vapor is also damaging, as are high levels of C3+ hydrocarbons. These issues are discussed in more detail in U.S. Pat. No. 5,407,466, columns 2-6, which patent is incorporated herein by reference.
Yet another challenging area is the separation of mixtures of light hydrocarbon vapors. For example, olefins, particularly ethylene and propylene, are important chemical feedstocks. About 17.5 million tons of ethylene and 10 million tons of propylene are produced in the United States annually, much as a by-product of petrochemical processing. Before they can be used, the raw olefins must be separated from mixtures containing saturated hydrocarbons and other components. Currently, separation of olefin/paraffin mixtures is usually carried out by distillation. The low relative volatilities of the components make this process costly and complicated; distillation columns are typically up to 300 feet tall and the process is very energy-intensive. More economical separation processes are needed. Using a membrane to separate olefins from paraffins is an alternative to distillation that has been considered. However, the separation is difficult because of the similar molecular sizes and condensabilities of the components, as well as the challenge of operating the membranes in a hydrocarbon-rich environment, and no material that can provide adequate performance with real vapor mixtures under pressure has been found.
Thus, the need remains for membranes that will provide and maintain adequate performance under the conditions of exposure to hydrocarbons, and particularly C3+ hydrocarbons, that are commonplace in refineries, chemical plants, or gas fields.
Films or membranes made from fluorinated polymers having a ring structure in the repeat unit are known. For example:
1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclose various perfluorinated polymers having repeating units of perfluorinated cyclic ethers, and cite the gas-permeation properties of certain of these, as in column 8, lines 48-60 of 4,910,276.
2. A paper entitled xe2x80x9cA study on perfluoropolymer purification and its application to membrane formationxe2x80x9d(V. Arcella et al., Journal of Membrane Science, Vol. 163, pages 203-209 (1999)) discusses the properties of membranes made from a copolymer of tetrafluoroethylene and a dioxole. Gas permeation data for various gases are cited.
3. European Patent Application 0 649 676 A1, to L""Air Liquide, discloses post-treatment of gas separation membranes by applying a layer of fluoropolymer, such as a perfluorinated dioxole, to seal holes or other defects in the membrane surface.
4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patent also discloses comparative data for membranes made from perfluoro(2-methylene-4-methyl- 1,3-dioxolane) polymer (Example XI).
5. A paper entitled xe2x80x9cGas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro- 1,3-dioxole/tetrafluoroethylenexe2x80x9d (I.Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133 (1996)) discusses the free volume and gas permeation properties of fluorinated dioxole/tetrafluoroethylene copolymers compared with substituted acetylene polymers. This reference also shows the susceptibility of this dioxole polymer to plasticization by organic vapors and the loss of selectivity as vapor partial pressure in a gas mixture increases (FIGS. 3 and 4).
Most of the data reported in the prior art references listed above are for permanent gases, carbon dioxide and methane, and refer only to measurements made with pure gases. The data reported in item 5 indicate that even these fluorinated polymers, which are characterized by their chemical inertness, appear to be similar to conventional hydrogen-separating membranes in their inability to withstand exposure to propane and heavier hydrocarbons.
The invention is a process for separating a gas from a gas mixture containing a C3+ hydrocarbon vapor. The separation is carried out by running a stream of the gas mixture across a membrane that is selective for the desired gas to be separated over the C3+ hydrocarbon vapor. The process results, therefore, in a permeate stream enriched in the desired gas and a residue stream depleted in that gas. The process differs from processes previously available in the art in that:
(i) the membranes are able to maintain useful separation properties in the presence of C3+ hydrocarbon vapor at high levels in the gas mixture, and
(ii) the membranes can recover from accidental exposure to liquid organic compounds.
To quantify these attributes, the membranes used in the process of the invention are characterized in terms of their selectivity before and after exposure to liquid hydrocarbons. Specifically, the membranes have a post-exposure selectivity for the desired gas over the C3+ hydrocarbon vapor, after exposure of the separation membrane to a liquid hydrocarbon, for example, toluene, and subsequent drying, that is at least about 60%, 65% or even 70% of a pre-exposure selectivity for the desired gas over the C3+ hydrocarbon vapor, the pre- and post-exposure selectivities being measured with a test gas mixture of the same composition and under like conditions.
At least the selective layer responsible for the gas discriminating properties of the membranes is made from an amorphous glassy polymer or copolymer. The polymer is fluorinated, generally heavily fluorinated, by which we mean having a fluorine:carbon ratio of atoms in the polymer of at least about 1:1. Preferably, the polymer is perfluorinated. Typically, the polymer is further characterized by a fractional free volume no greater than about 0.3 and by a glass transition temperature, Tg, of at least about 100xc2x0 C.
In a basic embodiment, the process of the invention includes the following steps:
(a) passing a gas mixture comprising a desired gas and a C3+ hydrocarbon vapor across the feed side of a separation membrane having a feed side and a permeate side, the separation membrane having a selective layer comprising a polymer having:
(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;
(ii) a fractional free volume no greater than about 0.3; and
(iii) a glass transition temperature of at least about 100xc2x0 C.; and the separation membrane being characterized by a post-exposure selectivity for the desired gas over the C3+ hydrocarbon vapor, after exposure of the separation membrane to liquid toluene and subsequent drying, that is at least about 65% of a pre-exposure selectivity for the desired gas over the C3+ hydrocarbon vapor, as measured pre- and post-exposure with a test gas mixture of the same composition and under like conditions;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in the desired gas compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in the desired gas compared to the gas mixture.
The permeating desired gas may be either a valuable gas that it is desired to retrieve as an enriched product, or a contaminant that it is desired to remove. Thus either the permeate stream or the residue stream, or both, may be the useful products of the process. Gases that may be separated from C3+ hydrocarbons by the process include, but are not limited to, hydrogen, nitrogen, oxygen, air, argon, carbon dioxide, methane, ethane, light olefins and light hydrocarbon isomers. Examples of C3+ hydrocarbon vapors from which the gas may be separated include, but are not limited to paraffins, both straight and branched, for example, propane, butanes, pentanes, hexanes; olefins and other aliphatic unsaturated organics, for example, propylene, butene; aromatic hydrocarbons, for example, benzene, toluene, xylenes; vapors of halogenated solvents, for example, methylene chloride, perchloroethylene; alcohols; ketones; and diverse other volatile organic compounds.
Various materials may be used for the polymeric selective layer to meet the characterizing requirements. These include, but are not limited to:
(i) polymers comprising a fluorinated dioxole monomer;
(ii) polymers comprising a fluorinated dioxolane monomer;
(iii) polymers comprising a fluorinated alkyl ether monomer;
(iv) perfluorinated polyimides;
(v) amorphous copolymers of tetrafluoroethylene.
Particularly preferred materials for the selective layer of the membrane used to carry out the process of the invention are copolymers of a fluorinated dioxole with tetrafluoroethylene. A specific most preferred material is such a copolymer having the structure: 
where x and y represent the relative proportions of the dioxole and the tetrafluoroethylene blocks, such that x+y=1.
Contrary to what would be expected from the data presented in the Pinnau et al. Journal of Membrane Science paper, we have unexpectedly found that membranes formed from such fluorinated dioxole/tetrafluoroethylene copolymers can withstand exposure to C3+ hydrocarbons well enough to provide useful separation capability for gas mixtures that include C3+ hydrocarbons. This resistance persists even when the C3+ hydrocarbons are present at high levels, such as 5%, 10%, 15% or even more.
A particularly important advantage of the invention is that the membranes can retain selectivity for desired gases, such as hydrogen, nitrogen, carbon dioxide, methane, or light olefin, even in the presence of streams rich in, or even essentially saturated with, C3+ hydrocarbons. This distinguishes these membrane materials from all other membrane materials previously used commercially for such separations. Membranes made from fluorinated dioxoles have been believed previously to behave like conventional membrane materials in suffering from debilitating plasticization in a hydrocarbon containing environment, to the point that they may even become selective for hydrocarbons over permanent gas even at moderate C3+ hydrocarbon partial pressures. We have discovered that this is not the case for the membranes taught herein. This unexpected result is achieved because the membranes used in the invention are unusually resistant to plasticization by hydrocarbon vapors.
The membranes are also resistant to contact with liquid hydrocarbons, in that they are able to retain their selectivity for hydrogen over methane after transient or even prolonged exposure to liquid aromatics, for example. This is a second beneficial characteristic that differentiates the processes of the invention from prior art processes. In the past, exposure of the membranes to liquid hydrocarbons frequently meant that the membranes were irreversibly damaged and had to be removed and replaced.
These unexpected and unusual attributes render the process of the invention useful in situations where it was formerly difficult or impractical for membrane separation to be used, or where membrane lifetimes were poor.
Because the preferred polymers are glassy and rigid, an unsupported film of the polymer may be usable in principle as a single-layer gas separation membrane. However, such layer will normally be far too thick to yield acceptable transmembrane flux, and in practice, the separation membrane usually comprises a very thin selective layer that forms part of a thicker structure, such as an asymmetric membrane or a composite membrane. The making of these types of membranes is well known in the art. If the membrane is a composite membrane, the support layer may optionally be made from a fluorinated polymer also, making the membrane a totally fluorinated structure and enhancing chemical resistance. The membrane may take any form, such as hollow fiber, which may be potted in cylindrical bundles, or flat sheets, which may be mounted in plate-and-frame modules or formed into spiral-wound modules.
The driving force for permeation across the membrane is the pressure difference between the feed and permeate sides, which can be generated in a variety of ways. The pressure difference may be provided by compressing the feedstream, drawing a vacuum on the permeate side, or a combination of both. The membrane is able to tolerate high feed pressures, such as above 200 psia, 300 psia, 400 psia or more. As mentioned above, the membrane is able to operate satisfactorily in the presence of C3+ hydrocarbons at high levels. Thus, the partial pressures of the hydrocarbons in the feed may be close to saturation. For example, depending on the mix of hydrocarbons and the temperature of the gas, the aggregate partial pressure of all C3+ hydrocarbons in the gas might be as much as 10 psia, 15 psia, 25 psia, 50 psia, 100 psia, 200 psia or more. Expressed as a percentage of the saturation vapor pressure at that temperature, the partial pressure of hydrocarbons, particularly C3+ hydrocarbons, may be 20%, 30%, 50% or even 70% or more of saturation.
The membrane separation process may be configured in many possible ways, and may include a single membrane unit or an array of two or more units in series or cascade arrangements. The processes of the invention also include combinations of the membrane separation process defined above with other separation processes, such as adsorption, absorption, distillation, condensation or other types of membrane separation.
In another aspect, the invention is a process for separating hydrogen from hydrocarbons in a multicomponent mixture containing at least hydrogen and one or more C3+ hydrocarbon vapors. Such a mixture might typically, but not necessarily, be found as a petrochemical plant or a refinery process or waste stream, such as streams from reformers, crackers, hydroprocessors and the like.
The process involves running a stream containing hydrogen across the feed side of a membrane that is selectively permeable to the hydrogen over the hydrocarbons in the stream. The hydrogen is concentrated in the permeate stream; the residue stream is thus correspondingly depleted of hydrogen. The process can separate hydrogen from methane, hydrogen from C2+ hydrocarbons, hydrogen from C3+ hydrocarbon vapors, or any combination of these.
In a basic embodiment, the process of the invention includes the following steps:
(a) passing a gas mixture comprising hydrogen and a C3+ hydrocarbon vapor across the feed side of a separation membrane having a feed side and a permeate side, the separation membrane having a selective layer comprising a polymer having:
(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;
(ii) a fractional free volume no greater than about 0.3; and
(iii) a glass transition temperature of at least about 100xc2x0 C.; and the separation membrane being characterized by a post-exposure selectivity for hydrogen over the C3+ hydrocarbon vapor, after exposure of the separation membrane to liquid toluene and subsequent drying, that is at least about 65% of a pre-exposure selectivity for hydrogen over the C3+ hydrocarbon vapor, as measured pre- and post-exposure with a test gas mixture of the same composition and under like conditions;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in hydrogen compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in hydrogen compared to the gas mixture.
The process differs from previous hydrogen/hydrocarbon separation processes in the nature of the membrane that is used. The membranes are, as described above, able to maintain useful separation properties in the presence of organic vapors at high activity, and able to recover from accidental exposure to liquid hydrocarbons.
The scope of the invention in this aspect is not intended to be limited to any particular gas streams, but to encompass any situation where a gas stream containing hydrogen and hydrocarbon gas is to be separated. The composition of the gas may vary widely, from a mixture that contains minor amounts of hydrogen in admixture with various hydrocarbon components, including relatively heavy hydrocarbons, such as C5-C8 hydrocarbons or heavier, to a mixture of mostly hydrogen, such as 80% hydrogen, 90% hydrogen or above, with methane and other very light components.
The process of the invention typically provides a selectivity, in mixtures containing multiple hydrocarbons including a C3+ hydrocarbon, for hydrogen over methane of at least about 10, for hydrogen over propane of at least about 50, and for hydrogen over n-butane of at least about 100. Frequently, the hydrogen/methane selectivity achieved is 20 or more, even in the presence of significant concentrations of C3+ hydrocarbons.
In yet another aspect, the invention is a process for separating olefins from paraffins, particularly propylene from propane. Such mixtures are found as olefin manufacturing effluent streams, and in various petrochemical plant streams, for example.
The process involves running a stream comprising propylene and propane across the feed side of a membrane that is selectively permeable to propylene. The propylene is concentrated in the permeate stream; the residue stream is thus correspondingly depleted of propylene.
In a basic embodiment, the process of the invention includes the following steps:
(a) passing a gas mixture comprising propylene and propane across the feed side of a separation membrane having a feed side and a permeate side, the membrane having a selective layer comprising a polymer having:
(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;
(ii) a fractional free volume no greater than about 0.3; and
(iii) a glass transition temperature of at least about 100xc2x0 C.; and the separation membrane being characterized by a post-exposure selectivity for propylene over propane, after exposure of the separation membrane to liquid toluene and subsequent drying, that is at least about 65% of a pre-exposure selectivity for propylene over propane as measured pre- and post-exposure with a test gas mixture of the same composition and under like conditions;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in propylene compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in propylene compared to the gas mixture.
The process typically provides a propylene/propane selectivity of at least about 2.5, and more preferably at least about 3, which can be sustained, even with streams composed entirely of C3+ hydrocarbons, over a range of pressures.
In yet another aspect, the invention is a process for separating carbon dioxide from methane and other hydrocarbons. Such a mixture might be encountered during the processing of natural gas, of associated gas from oil wells, or of certain petrochemical streams, for example.
The process involves running a stream containing carbon dioxide across the feed side of a membrane that is selectively permeable to the carbon dioxide over methane and the other hydrocarbons in the stream. The carbon dioxide is concentrated in the permeate stream; the residue stream is thus correspondingly depleted of carbon dioxide.
The process of the invention typically provides a selectivity, in mixtures containing multiple hydrocarbons including a C3+ hydrocarbon, for carbon dioxide over methane of at least about 5, even at high carbon dioxide activity. Frequently, the carbon dioxide/methane selectivity achieved is 10 or more, and may be as much as 15 or more, even in the presence of significant concentrations of C2+ hydrocarbons.
Other separation processes that can be carried out within the scope of the invention include, but are not limited to, separation of other permanent gases, for example, nitrogen, oxygen, air or argon, from organics; separation of methane from C3+ organics; and separation of isomers from one another.
It is an object of the present invention to provide a membrane-based process for separation of gases from gas mixtures containing C3+ hydrocarbon vapors. Additional objects and advantages of the invention will be apparent from the description below to those of ordinary skill in the art.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.